Mass spectrometers separate ions based on the charge-to-mass ratio of the ions. A compound that is to be analyzed utilizing a mass spectrometer must first be ionized. The ionization process can also lead to the breakup of the compound into a number of ions whose relative abundances in the mass spectrometer spectrum are indicative of the type of compound. Ionization sources that operate at atmospheric pressure are useful in the analysis of many compounds of interest. However, such sources require some form of interface between the ion source and the vacuum of the mass spectrometer to preserve the vacuum within the mass spectrometer.
Typically, the interface is a capillary tube having one end that samples ions from the ion source and the other end in the mass spectrometer. The efficiency with which ions are moved from the ion source to the mass spectrometer through such a tube is low due to the size of the tube and losses in the tube. In addition, if the tube is constructed from a dielectric, ions that are absorbed onto the surface of the tube can create a space charge that hinders the transit of ions of the same charge through the tube.
Alternatively, a small orifice is used to limit the flow between the source and the mass spectrometer. In this case, the efficiency of ion collection is also low because the diameter of the orifice must be small to preserve the vacuum within the mass spectrometer.
Further, the sample of interest is often dissolved in a solvent that is vaporized in the ion source. The carrier solvent and gases used in the ion source to vaporize the solvent can result in unwanted ions that lead to undesirable background species in the mass spectrometer.